Method for producing nitride fluorescent material, nitride fluorescent material and light emitting device

ABSTRACT

Disclosed are a production method for a nitride fluorescent material, a nitride fluorescent material and a light emitting device. The production method is for producing a nitride fluorescent material that has, as a fluorescent material core, a calcined body having a composition containing at least one element M a  selected from the group consisting of Sr, Ca, Ba and Mg, at least one element M b  selected from the group consisting of Li, Na and K, at least one element M c  selected from the group consisting of Eu, Ce, Tb and Mn, and Al, and optionally Si, and N, and the method includes preparing a calcined body having the above-mentioned composition, bringing the calcined body into contact with a fluorine-containing substance, and subjecting it to a first heat treatment at a temperature of 100° C. or higher and 500° C. or lower to form a fluoride-containing first film on the calcined body, and forming on the calcined body, a second film that contains a metal oxide containing at least one metal element M2 selected from the group consisting of Si, Al, Ti, Zr, Sn and Zn and subjecting it to a second heat treatment at a temperature in a range of higher than 250° C. and 500° C. or lower.

CROSS-REFERENCE TO RELATED PAYENT APPLICATION

This is a divisional application of U.S. Patent application Ser. No.16/117,514, filed Aug. 30, 2018, which claims priority to JapanesePatent Application No. 2017-167127, filed on Aug. 31, 2017, the entiredisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND Technical Field

The present disclosure relates to a method for producing a nitridefluorescent material, a nitride fluorescent material and a lightemitting device. In this specification, the “fluorescent material” isused in the same meaning as a “fluorescent phosphor”.

Description or Related Art

As a fluorescent material that emits light in red and is used in a lightemitting device constructed by combining a light emitting diode(hereinafter referred to as “LED”) and a fluorescent material, there isa nitride fluorescent material having a composition represented bySrLiAl₃N_(4:) Eu (hereinafter may be referred to as “SLAN fluorescentmaterial”). For example, Patent Literature 1 (PTL 1) and Non PatentLiterature 1 (NPL 1) (Philipp Pust et al., “Narrow-band red-emittingSr[LiAl₃N₄]:Eu²⁺ as a next-generation LED-phosphor material” NatureMaterials, NMAT4012, VOL13 September 2014) disclose a SLAN fluorescentmaterial having a light emission peak wavelength at around 650 nm.

As disclosed in NPL1, a SLAN phosphor is, for example, produced suchthat powders of raw materials including lithium aluminum hydride(LiAlH₄), aluminum nitride (AlN), strontium hydride (SrH₂), and europiumfluoride (EuF₃) are weighed in a stoichiometric ratio so that 0.4% bymol Eu is mixed. The mixture is placed in a crucible and calcined in amixed gas atmosphere of hydrogen and nitrogen under an atmosphericpressure at a temperature of 1000° C. for two hours.

Citation List Patent Literature

PTL1: Japanese Laid-open Patent Publication No.2015-526532

Non Patent Literature

NPL1: Philipp Pust et al. “Narrow-band red-emitting Sr[LiAl₃N₄]:Eu²⁺as anext-generation LED-phosphor material” Nature Materials, NMAT4012, vol.13, September 2014.

SUMMARY

However, it is known that the SLAN fluorescent material readily degradedepending on external environments. It is desired to further improve thedurability of a light emitting device that uses such a SLAN fluorescentmaterial.

An object of the present disclosure is to provide a method for producinga nitride fluorescent material whose degradation by externalenvironments is suppressed, and to provide a light emitting deviceexcellent in durability.

The present disclosure includes the following embodiments.

A first embodiment of the present disclosure is a method for producing anitride fluorescent material that contains, as a fluorescent materialcore, a calcined body having a composition containing at least oneelement M^(a) selected from the group consisting of Sr, Ca, Ba and Mg,at least one element M^(b) selected from the group consisting of Li, Naand K, at least one element M^(c) selected from the group consisting ofEu, Ce, Tb and Mn, and Al, optionally Si, and N; the method includingpreparing a calcined body having the above-mentioned composition,bringing the calcined body into contact with a fluorine-containingsubstance, and subjecting the calcined body to a first heat treatment ata temperature in a range of 100° C. or higher and 500° C. or lower toform a fluoride-containing first film on the calcined body, and formingon the calcined body, a second film that contains a metal oxidecontaining at least one metal element M2 selected from the groupconsisting of Si, Al, Ti, Zr, Sn and Zn and subjecting the calcined bodyto a second heat treatment at a temperature in a range of higher than250° C. and 500° C. or lower.

A second embodiment of the present disclosure is a nitride fluorescentmaterial having, on the surface of a fluorescent material corecontaining at least one element M^(a) selected from the group consistingof Sr, Ca, Ba and Mg, at least one element M^(b) selected from the groupconsisting of Li, Na and K, at least one element M^(c) selected from thegroup consisting of Eu, Ce, Tb and Mn, and Al, and optionally Si, and N,a first film containing a fluoride, and a second film containing a metaloxide that contains at least one metal element M2 selected from thegroup consisting of Si, Al, Ti, Zr, Sn and Zn, wherein:

the first film contains the metal element M2 and the amount of the metalelement M2 contained in the second film is larger than the amount of themetal element M2 contained in the first film.

A third embodiment of the present disclosure is a light emitting deviceincluding the above-mentioned nitride fluorescent material and anexcitation light source.

According to the embodiments of the present disclosure, there can beprovided a method for producing a nitride fluorescent material whosedegradation by external environments is suppressed, and a light emittingdevice excellent in durability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view showing one example of alight emitting device according to the present disclosure.

FIG. 2 is a diagram showing light emission spectra of nitridefluorescent materials of an Example and Comparative Examples in thepresent disclosure.

FIG. 3 is a SEM micrograph of a secondary electron image of a nitridefluorescent material of Example 1 in the present disclosure.

FIG. 4 is a SEM micrograph of a reflected electron image of a crosssection of a nitride fluorescent material of Example 2 in the presentdisclosure.

FIG. 5 is a SEM micrograph of a secondary electron image of a nitridefluorescent material of Comparative Example 1 in the present disclosure.

FIG. 6 is a SEM micrograph of a secondary electron image of the nitridefluorescent material of Comparative Example 2 in the present disclosure.

FIG. 7 is a SEM micrograph of a secondary electron image of a nitridefluorescent material of Comparative Example 3 in the present disclosure.

FIG. 8 is a SEM micrograph of a reflected electron image of a crosssection of the nitride fluorescent material of Comparative Example 3 inthe present disclosure.

FIG. 9 is a SEM micrograph of a reflected electron image of a crosssection of a nitride fluorescent material of Comparative Example 4 inthe present disclosure.

DETAILED DESCRIPTION

The method for producing a nitride fluorescent material, the nitridefluorescent material and the light emitting device to the presentinvention (hereinafter may be simply referred to as “the productionmethod for a fluorescent material and others”) are described hereinunderon the basis of embodiments. The embodiments as shown below are merelyto exemplify the production method for a fluorescent material and othersfor the purpose of embodying the technical concept of the presentinvention, and the present invention is not limited to the productionmethod for a fluorescent material and others shown below. In thisdescription, the relationship between the color names and thechromaticity coordinates, the relationship between the wavelength rangeof light and the color names of monochromic light are in accordance withJIS Z8110.

Method for Producing Nitride Fluorescent Material

The first embodiment of the present disclosure is a method for producinga nitride fluorescent material that contains, as a fluorescent materialcore, a calcined body having a composition containing at least oneelement M^(a) selected from the group consisting of Sr, Ca, Ba and Mg,at least one element M^(b) selected from the group consisting of Li, Naand K, at least one element M^(c) selected from the group consisting ofEu, Ce, Tb and Mn, and Al, and optionally Si, and N, and the methodincludes preparing a calcined body having the above-mentionedcomposition, bringing the calcined body into contact with afluorine-containing substance, and subjecting the calcined body to afirst heat treatment at a temperature in a range of 100° C. or higherand 500° C. or lower to form a fluoride-containing first film on thecalcined body, and forming, on the calcined body, a second film thatcontains a metal oxide containing at least one metal element M2 selectedfrom the group consisting of Si, Al, Ti, Zr, Sn and Zn and subjectingthe calcined body to a second heat treatment at a temperature in a rangeof higher than 250° C. and 500° C. or lower.

Preparation of Calcined Body

The calcined body to be the fluorescent material core may be any one,not specifically limited, having a composition containing at least oneelement M^(a) selected from the group consisting of Sr, Ca, Ba and Mg,at least one element M^(b) selected from the group consisting of Li, Naand K, at least one element M^(c) selected from the group consisting ofEu, Ce, Tb and Mn, and Al, and optionally Si, and N.

Preferably, the fluorescent material core has a composition representedby the following formula (I):

M^(a) _(v)M^(b) _(w)M^(c) _(x)Al_(3-y)Si_(y)N_(z)  (I)

wherein M^(a) represents at least one element selected from the groupconsisting of Sr, Ca, Ba and Mg, M^(b) represents at least one elementselected from the group consisting of Li, Na and K, M^(c) represents atleast one element selected from the group consisting of Eu, Ce, Tb andMn, v, w, x, y and z each independently represent a number satisfying0.80≤v≤1.05, 0.80≤w≤1.05, 0.001<x≤0.1, 0≤y≤0.5, and 3.0≤z≤5.0.

Preferably, from the viewpoint of attaining a high light emissionintensity, the element M^(a) in the formula (I) for the fluorescentmaterial core contains at least one of Sr and Ca. In the case where theelement M^(a) contains at least one of Sr and Ca, the total molar ratioof Sr and Ca contained in the element M^(a) is, for example, 85 mol % ormore, and preferably 90 mol % or more.

Preferably, from the viewpoint of stability of crystal structures, theelement M^(b) in the formula (I) contains at least Li. In the case wherethe element M^(b) in the formula (I) contains Li, the molar ratio of Licontained in M^(b) is, for example, 80% by mol or more, and preferably90% by mol or more.

Regarding the values of parameters v, w and x in the formula (I), theparameter v is, from the viewpoint of the stability of crystalstructures, preferably 0.80 or more and 1.05 or less, more preferably0.90 or more and 1.03 or less. The parameter w is, from the viewpoint ofstability of crystal structures, preferably 0.80 or more and 1.05 orless, more preferably 0.90 or more and 1.03 or less. The parameter x isan activation amount of at least one element selected from the groupconsisting of Eu, Ce, Tb and Mn, and may be appropriately selected so asto attain the desired characteristics. More preferably, the parameter xis a number satisfying 0.001<x≤0.020 and is even more preferably anumber satisfying 0.002≤x≤0.015.

The calcined body to be the fluorescent material core may be produced bymixing raw materials so that the composition may contain the elementM^(a), the element M^(b), the element M^(c) and Al, and optionally Siand N, and then calcining the resultant raw material mixture in anatmosphere containing nitrogen gas, for example, at a temperature of1000° C. or higher and 1300° C. or lower under a pressure of 0.2 MPa ormore and 200 MPa or less. For the method of producing the calcined bodyto be the fluorescent material core, for example, the method describedin Japanese Patent Application No. 2016-193960 may be referred to.

The raw material mixture for use for the calcined body to be thefluorescent material core is not specifically limited in point of thematerials contained in the raw material mixture so far as the mixturegives the above-mentioned calcined body. For example, the raw materialmixture may contain at least one selected from the group consisting ofsimple substances of metal elements and metal compounds thereof toconstitute the above-mentioned composition.

Specifically, an M^(a) compound containing the element M^(a) includesSrN₂, SrN, Sr₃N₂, SrH₂, SrF₂, Ca₃N₂, CaH₂, CaF₂, Ba₃N₂, BaH₂, BaF₂,Mg₃N₂, MgH₂, and MgF₂, and at least one selected from the group of theseis preferred. In the case where SrF₂ is used as a raw material, it mayfunction not only as the M^(a) compound but also as a flux. As the M^(a)compound, compounds such as imide compounds, amide compounds may also beused.

An M^(b) compound containing the element M^(b) preferably contains atleast Li, and is more preferably at least one of Li nitrides andhydrides. In the case where the second compound contains Li, a part ofLi may be substituted with Na and/or K, and may contain any other metalelement constituting the nitride fluorescent material. Specifically, theM^(b) compound containing Li includes Li₃N, LiN₃, LiH, and LiAlH₄.

An Al compound containing Al may be a compound containing substantiallyAl alone as the metal element, or may be a compound where a part of Alis substituted with an element selected from the Group 13 elements Gaand In, and transition metals of the 4th Period V, Cr, Co and others, ormay be a metal compound containing any other element than Al toconstitute the nitride fluorescent material, such as Li, in addition toAl. Specifically, the Al compound includes AlN, AlH₃, AlF₃, and LiAlH₄,and is preferably at least one selected from the group of these.

In the case where a part of Al in the composition of the calcined bodycontains Si, an Si-containing compound may be used in the raw materialmixture. As the Si-containing compound, at least one selected from thegroup consisting of SiO₂, Si₃N₄, SiC, and SiCl₄ may be used.

An M^(c) compound containing the element M^(c) contains Eu, Ce, Tb or Mnas an activator. In the case where the element M^(c) is Eu, a part of Eucontained in the M^(c) compound may be substituted with Sc, Y, La, Ce,Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu. It is considered thatwhen a part of Eu is substituted with any other element, the otherelement may act, for example, as a coactivator. By coactivating thenitride fluorescent material with two or more kinds of activatingelements, the light emission characteristics of the material can becontrolled.

In the case where the element M^(c) is Eu, specifically, the M^(c)compound includes Eu₂O₃, EuN, and EuF₃, and at least one selected fromthe group of these is preferably used.

The raw material mixture may contain a flux. When the raw materialmixture contains a flux, the reaction between raw materials can bepromoted and further solid-phase reaction can run on more uniformly, andtherefore a calcined body for giving a nitride fluorescent materialhaving a large particle size and having more excellent light emissioncharacteristics can be produced.

Raw materials are, for example, after each weighed in a glove box in aninert atmosphere, mixed to give a raw material mixture.

The raw material mixture is calcined in a nitrogen atmosphere. Forcalcining, for example, a gas-pressurized electric furnace may be used.The calcining temperature is preferably within a range of 1000° C. orhigher and 1400° C. or lower. The calcining temperature is morepreferably 1000° C. or higher and 1300° C. or lower, even morepreferably 1100° C. or higher and 1300° C. or lower. When the calciningtemperature is low, the intended fluorescent compound could hardly beformed, but when the calcining temperature is high, the fluorescentmaterial compound may decompose to detract from the light emissioncharacteristics thereof. The calcining may be carried out in two-stagecalcining (multistage calcining) in such a manner that the first-stagecalcining is carried out at 800° C. or higher and 1000° C. or lower,then the system is gradually heated, and the second-stage calcining iscarried out at 1000° C. or higher and 1400° C. or lower.

Preferably, the calcining atmosphere is a nitrogen gas-containingatmosphere, and may be an atmosphere containing at least one selectedfrom the group consisting of hydrogen, argon, carbon dioxide, carbonmonoxide, ammonia in addition to nitrogen gas. Preferably, the ratio ofthe nitrogen gas in the calcining atmosphere is 70% by volume or more,more preferably 80% by volume or more.

Preferably, the calcining is carried out in a pressurized atmosphere at0.2 MPa or more and 200 MPa or less. The calcined body having theintended composition may readily decompose at a higher temperature, butin a pressurized atmosphere, the decomposition may be suppressed toattain more excellent light emission characteristics. The gauge pressurein the pressurized atmosphere is more preferably 0.2 MPa or more and 1.0MPa or less, even more preferably 0.8 MPa or more and 1.0 MPa or less.By increasing the atmosphere gas pressure in calcining, thedecomposition during calcining can be suppressed and a calcined bodyhaving the intended composition is easy to produce.

The calcining time may be appropriately selected depending on thecalcining temperature, the gas pressure, etc. The calcining time is, forexample in range of 0.5 hours or more and 20 hours or less, and ispreferably in a range of 1 hour or more and 10 hours or less.

By calcining, a calcined body having the intended composition can beobtained.

Formation of First Film

The production method for a nitride fluorescent material of theembodiment of the present disclosure includes bringing the calcined bodyto be a fluorescent material core into contact with afluorine-containing substance and subjecting it to a first heattreatment at a temperature in a range of 100° C. or higher and 500° C.or lower to form a fluoride-containing first film on the calcined body.For the method for forming the first film on the calcined body to be afluorescent material core, for example, the method described in JapanesePatent Application No. 2016-193960 may be referred to.

Through the process of bringing the calcined body into contact with afluorine-containing substance followed by first heat treatment in arange of 100° C. or higher and 500° C. or lower, a fluoride-containingfirst film is formed on the calcined body.

Fluorine-Containing Substance

The fluorine-containing substance includes a fluorine gas (F₂) and afluorine compound. The fluorine compound is preferably at least oneselected from the group consisting of CHF₃, CF₄, NH₄HF₂, NH₄F, SiF₄,KrF₂, XeF₂, XeF₄ and NF₃. The fluorine-containing substance is morepreferably a fluorine gas (F₂) or an ammonium fluoride (NH4F). Thetemperature in the environment where the calcined body is brought intocontact with the fluorine-containing substance may be room temperature(20° C.±5° C.) to a temperature lower than the first heat treatmenttemperature, or may be the first heat treatment temperature.

In the case where the fluorine-containing substance is in a solid stateor a liquid state at room temperature, preferably, thefluorine-containing substance is brought into contact with the calcinedbody in an amount in a range of 1% by mass or more and 10% by mass orless relative to 100% by mass of the total amount of the calcined bodyand the fluorine-containing substance, more preferably in a range of 2%by mass or more and 8% by mass or less, even more preferably in a rangeof 3% by mass or more and 7% by mass or less. Accordingly, it ispresumed that a layer of a fluorine-containing compound may be readilyformed on the surface or in the vicinity of the surface of the calcinedbody.

In the case where the fluorine-containing substance is gaseous, thecalcined body may be arranged in an atmosphere containing thefluorine-containing substance for contact therebetween, or the firstheat treatment may be carried out in an atmosphere containing afluorine-containing substance. The F₂ concentration in the atmosphere ispreferably in a range of 2% by volume or more and 25% by volume or less,more preferably in a range of 5% by volume or more and 20% by volume orless. When the F₂ concentration in the atmosphere is lower than apredetermined level, desired durability could not be attained. On theother hand, when the F₂ concentration is higher than a predeterminedlevel, even the base body of the fluorescent material would befluorinated to significantly lower the light emission intensity of thefluorescent material.

First Heat Treatment

The calcined body is brought into contact with a fluorine-containingsubstance and then subjected to first heat treatment in a range of 100°C. or higher and 500° C. or lower. Through the first heat treatment, afluoride-containing first film is formed on the calcined body.

Preferably, the first heat treatment is carried out in an inert gasatmosphere. In this description, an inert gas atmosphere means anatmosphere that contains argon, helium, nitrogen as the main componenttherein and has a concentration of oxygen contained therein of 15% byvolume or less. The inert gas atmosphere may contain oxygen as anunavoidable impurity. The concentration of oxygen in the inert gasatmosphere is preferably 10% by volume or less, more preferably 5% byvolume or less, even more preferably 1% by volume or less. This isbecause, when the oxygen concentration is higher than a predeterminedlevel, the particles of the fluorescent material would be too muchoxidized. In consideration of safety, the first heat treatment ispreferably carried out in an atmosphere containing an inert gas and afluorine-containing substance.

The first heat treatment temperature is in a range of 100° C. or higherand 500° C. or lower, preferably in a range of 120° C. or higher and450° C. or lower, more preferably in a range of 150° C. or higher and400° C. or lower, even more preferably in a range of 150° C. or higherand 350° C. or lower.

When the first heat treatment temperature is lower than a predeterminedtemperature, a fluoride-containing first film could hardly formed on thecalcined body. On the other hand, when the first heat treatmenttemperature is higher than a predetermined temperature, the crystalstructure of the calcined body would be readily broken.

Though not specifically limited, the first heat treatment time ispreferably in a range of 1 hour or more and 10 hours or less, morepreferably in a range of 2 hours or more and 8 hours or less. When thefirst heat treatment time is in a range of 1 hour or more and 10 hoursor less, a fluoride-containing first film may be formed on the surfaceor in the vicinity of the surface of the calcined body through the firstheat treatment in contact between the calcined body and afluorine-containing substance.

After the first heat treatment, post heat treatment may be carried outat a temperature higher than in the first heat treatment. The post heattreatment temperature is preferably higher than that in the first heattreatment, and is preferably in a range of 200° C. or higher and 500° C.or lower, more preferably in a range of 250° C. or higher and 450° C. orlower, even more preferably in a range of 300° C. or higher and 400° C.or lower. By carrying out the post heat treatment after the first heattreatment, a fluoride-containing, tough first film that secures highbonding to the surface of the calcined body can be formed.

Though not specifically limited, the post heat treatment time ispreferably in a range of 1 hour or more and 15 hours or less, morepreferably in a range of 2 hours or more and 12 hours or less, even morepreferably in a range of 3 hours or more and 10 hours or less.Preferably, the post heat treatment is carried out in air or in an inertgas atmosphere.

After the first heat treatment, a fluoride-containing first film thatcontains at least one element constituting the calcined body to be thefluorescent material core is formed on the surface of the calcined body.Preferably, the first film has a first layer and a second layer in thatorder from the side of the fluorescent material core. Preferably, thefirst layer contains a fluoride that differs from the composition of thesecond layer. When the first film has the first layer and the secondlayer, it functions as a double-layer protective film to protect thefluorescent material core to thereby prevent the core from beingdegraded by external environments. In the case where the nitridefluorescent material, in which the first layer and the second layer ofthe first film function as a double-layer protective film for thefluorescent material core, is used in a light emitting device, thedurability of the light emitting device can be thereby improved.Preferably, the first layer and the second layer each contain a fluoridehaving a composition that contains the element M^(a), fluorine and Al.Preferably, the fluoride contained in the first layer has a compositionhaving a larger molar ratio of Al relative to the element M^(a) than inthe fluoride contained in the second layer. When the fluoride containedin the first layer has a composition having a larger molar ratio of Alrelative to the element M^(a) than in the fluoride contained in thesecond layer, it is presumed that the fluorescent material core and thefirst layer may strongly bond to each other.

Preferably, the first heat treatment is carried out before the formationof the second film to be mentioned hereinunder. The nitride fluorescentmaterial may be readily influenced by oxygen, heat and moisture,depending on the crystal structure of the fluorescent material core, andtherefore preferably, the first film is formed by the first heattreatment and then, while the calcined body is protected with the firstfilm, the second film is formed, and further the second heat treatmentis carried out.

The first heat treatment may also be carried out after the second heattreatment to be mentioned hereinunder. In the case where the first heattreatment is carried out through contact between the calcined body and afluoride-containing substance after the second heat treatment, as thecase may be, a fluoride-containing film that contains at least one metalelement M2 selected from the group consisting of Si, Al, Ti, Zr, Sn andZn, and fluorine may be formed.

Formation of Second Film

The production method for a nitride fluorescent material of theembodiment of the present disclosure includes forming a second filmcontaining a metal oxide that contains the above-mentioned element M₂,on the calcined body, and subjecting it to a second heat treatment at atemperature in a range of higher than 250° C. and 500° C. or lower.

As one example of the second film formation method, preferably, thecalcined body is brought into contact with a solution containing a metalalkoxide, and through hydrolysis of the metal alkoxide andpolycondensation, a second film containing a metal oxide is formed. Morepreferably, the calcined body having the first film is brought intocontact with a metal alkoxide-containing solution, and throughhydrolysis of the metal alkoxide and polycondensation, a second filmcontaining a metal oxide is formed. Preferably, the second film isformed in a sol-gel process using a metal alkoxide. The metal alkoxidepreferably contains at least one metal element M2 selected from Si, Al,Ti, Zr, Sn and Zn.

Preferably, the metal alkoxide is a silane compound having 2 or morealkoxyl groups, and specifically includes methyltrimethoxysilane,methyltriethoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, propyltrimethoxysilane, p ropyltriethoxysilane, tetramethoxysilane,tetraethoxysilane, tetrap ropoxysilane, titanium tetrapropoxide,titanium tetrabutoxide, aluminum triethoxide, aluminum tripropoxide,aluminum tributoxide, zirconium tetrapropoxide, zirconium tetrabutoxide,tin tetrabutoxide, zinc tetrapropoxide, and zinc tetrabutoxide. Inconsideration of workability and easy availability, the metal alkoxideis preferably tetraethoxysilane.

In consideration of operability, preferably, the metalalkoxide-containing solution contains an organic solvent.

The organic solvent contained in the metal alkoxide-containing solutionis preferably a polar organic solvent, and examples thereof includeethyl acetate, tetrahydrofuran, N,N-diethylformamide, dimethylsulfoxide, alcohols having a linear or branched alkyl group with 1 to 8carbon atoms; carboxylic acids such as formic acid, acetic acid, etc.;ketones such as acetone, etc. The polar organic solvent is preferably alower alcohol having a linear or branched alkyl group with 1 to 3 carbonatoms, or a ketone. More preferably, the polar organic solvent isethanol or ketone having a relative permittivity of 18 to 33.Specifically, more preferably, the solvent is at least one selected fromthe group consisting of methanol (relative permittivity 33), ethanol(relative permittivity 24), 1-propanol (relative permittivity 20),2-propanol (relative permittivity 18) and acetone (relative permittivity21).

When the metal alkoxide-containing solution contains an acid or alkalicatalyst, the hydrolysis speed of the metal alkoxide may be therebyaccelerated. Examples of the acid or alkali solution to be the catalystinclude a hydrochloric acid solution and an ammonia solution.

The calcined body is brought into contact with the above-mentioned metalalkoxide-containing solution for hydrolysis of the metal alkoxidefollowed by polycondensation thereby forming a second film containing anoxide of the metal element M2 as the main component. For example, in thecase where the metal alkoxide is tetraethoxysilane (Si(OC₂H₅)₄), thecalcined body is brought into contact with a solution containingtetraethoxysilane (Si(OC₂H₅)₄) and the tetraethoxysilane is hydrolyzedto form orthosilicic acid (Si(OH)₄), and then dehydration reaction runson though polycondensation of orthosilicic acid (Si(OH)₄) to form asecond film containing silica (SiO₂) as the main component. The secondfilm contains silica (SiO₂) formed through the above-mentionedpolycondensation, and the second film may contain a silicon compoundwhere a hydroxyl group (OH) may partly remain therein.

In the case where the second film is formed on the calcined body,preferably, the second film is formed on the calcined body having thefirst film. The calcined body to be a fluorescent material core isreadily influenced by oxygen, heat and moisture owing to the crystalstructure. Therefore, in the case where the second film is formed on thecalcined body having the first film according to a sol-gel process,preferably, the first film has been formed on the surface of thecalcined body through the first heat treatment and then the second filmis formed thereon according to a sol-gel process.

Second Heat Treatment

The calcined body on which the second film containing a metal oxide hasbeen formed is subjected to second heat treatment in a temperature rangeof higher than 250° C. and 500° C. or lower. Through the second heattreatment, dehydration runs on in the second film containing a hydroxylgroup (OH) to give a tough film. In the case where the second filmcontaining a metal oxide is formed on the calcined body having thefluoride-containing first film, oxygen contained in the metal oxide inthe second film acts on the first film therefore resulting in that themetal element M2 is also contained in the first film. In the nitridefluorescent material obtained after the second heat treatment, theamount of the metal element M2 contained in the second film is largerthan that of the metal element M2 contained in the first film. Since themetal element M2 is contained also in the first film, it is presumedthat the fluoride contained in the first film may react with the metaloxide contained in the second film through the second heat treatmentwhereby dehydration may run on in the second film containing a hydroxylgroup (OH), and as a result, a part of the metal element M2 contained inthe second film may bond with the element (for example, Sr or Al) in thefluoride in the first film via oxygen so that the metal element M2 mayalso be contained in the first film. In the nitride fluorescent materialobtained according to the production method of the embodiment of thepresent disclosure, the first film and the second film function as adouble-layer protective film to prevent the nitride fluorescent materialfrom degrading in external environments, thereby further improving thedurability of a light emitting device containing the nitride fluorescentmaterial.

When the second film containing a metal oxide is formed on the firstfilm-having calcined body and then subjected to second heat treatment,as the case may be, oxygen contained in the metal oxide in the secondfilm may act also on the first film and fluorine may be contained alsoin the second film. Preferably, in the nitride fluorescent materialobtained after the second heat treatment, the amount of fluorinecontained in the first film is larger than the amount of fluorinecontained in the second film.

When the first film has a first layer and a second layer, and when ametal oxide-containing second film is formed on the first film-carryingcalcined body and then subjected to second heat treatment, the firstlayer of the first film arranged on the side of the calcined body to bethe fluorescent material core comes to have a composition containing theelement M^(a), fluorine, Al and oxygen, the second layer of the firstfilm arranged on the side of the second film comes to have a compositioncontaining fluorine, the element M^(a), Al, the metal element M2 andoxygen in an amount larger than that in the first layer. It is presumedthat the second layer arranged on the side of the second film may reactwith the metal oxide contained in the second film, and the element (forexample, the element M^(a) or Al) contained in the second layer of thefirst film, and the metal element M2 contained in the second film maybond to each other via oxygen to form a stable structure, and thereforeit is considered that the function to protect the fluorescent materialcore may be thereby improved.

As shown in the SEM micrograph of a reflected electron image of a crosssection of a nitride fluorescent material in FIG. 4 to be mentionedhereinunder, by forming a second film after the first treatment followedby the second heat treatment, a nitride fluorescent material is obtainedin which three layers of the first layer of the first film, the secondlayer of the first film and the second film are formed on the surface ofthe fluorescent material core. When the nitride fluorescent material hasthe first film having a double-layer structure and the second film, thefilm of these three layers functions as a protective film to prevent thenitride fluorescent material from degrading in external environments,and therefore the durability of the light emitting device using thenitride fluorescent material can be thereby improved.

The nitride fluorescent material is readily influenced by externalenvironments, and therefore, for example, when the second film is formedaccording to a sol-gel process, preferably, after the first film hasbeen formed on the surface of the fluorescent material core through thefirst heat treatment, the second film is formed, and thereafter thesecond heat treatment is carried out.

In the case where the second film is formed after the first heattreatment and thereafter the second heat treatment is carried out, thesecond film can be formed while the calcined body is protected with thefluoride-containing first film, and thereafter the second heat treatmentis further carried out. Accordingly, the calcined body can be preventedfrom degrading in the production process.

The second heat treatment temperature is in a range of higher than 250°C. and 500° C. or lower, preferably in a range of higher than 250° C.and 450° C. or lower, more preferably in a range of 300° C. or higherand 400° C. or lower.

When the second heat treatment temperature is 250° C. or lower, oxygenin the metal oxide contained in the second film could hardly act on thefirst film since the temperature is low, and if so, the metal oxidecontained in the second film could hardly bond to the element containedin the first film via oxygen and the function of the protective film maylower. On the other hand, when the second heat treatment temperature ishigher than 500° C., the crystal structure of the calcined body would bereadily broken.

Preferably, the second heat treatment is carried out in air or in aninert gas atmosphere.

Though not specifically limited, the second heat treatment time ispreferably in a range of 1 hour or more and 20 hours or less, morepreferably in a range of 2 hours or more and 15 hours or less, even morepreferably in a range of 3 hours or more and 10 hours or less. When thesecond heat treatment time is in a range of 1 hour or more and 20 hoursor less, the heat treatment would not have an influence on the structureof the fluorescent material core and a second film containing a metaloxide and having a small amount of a hydroxyl group (OH) therein may beformed.

The second heat treatment may be carried out before the first heattreatment.

In the case where a metal oxide-containing second film is formed and thesecond heat treatment is carried out before the first heat treatment, itis presumed that a compound containing at least a metal element M2 andfluorine may be formed in the second film though the first heattreatment of bringing the calcined body into contact with afluorine-containing substance.

Post-Treatment

The production method for a nitride fluorescent material of theembodiment of the present disclosure may include a post-treatment ofcrushing treatment, grinding treatment, classification treatment for theresultant nitride fluorescent material after the first heat treatmentand the second heat treatment.

Nitride Fluorescent Material

The nitride fluorescent material of one aspect of the present inventionhas, on the surface of a fluorescent material core containing at leastone element M^(a) selected from the group consisting of Sr, Ca, Ba andMg, at least one element M^(b) selected from the group consisting of Li,Na and K, at least one element M^(c) selected from the group consistingof Eu, Ce, Tb and Mn, and Al, and optionally Si, and N, a first filmcontaining a fluoride, and a second film containing a metal oxide thatcontains at least one metal element M2 selected from the groupconsisting of Si, Al, Ti, Zr, Sn and Zn, wherein the first film containsthe metal element M2 and the amount of the metal element M2 contained inthe second film is larger than the content of the metal element M2contained in the first film.

The nitride fluorescent material is preferably one produced according tothe production method of the embodiment of the present disclosure.

Fluorescent Material Core

Preferably, the fluorescent material core in the nitride fluorescentmaterial has a composition represented by the following formula (I):

M^(a) _(v)M^(b) _(w)M^(c) _(x)Al_(3-y)Si_(y)N_(z)  (I)

wherein M^(a) represents at least one element selected from the groupconsisting of Sr, Ca, Ba and Mg, M^(b) represents at least one elementselected from the group consisting of Li, Na and K, M^(c) represents atleast one element selected from the group consisting of Eu, Ce, Tb andMn, v, w, x, y and z each independently represent a number satisfying0.80≤v≤1.05, 0.80≤w≤1.05, 0.001<x≤0.1, 0≤y≤0.5, and 3.0≤z≤5.0.

Preferably, from the viewpoint of attaining a high light emissionintensity, the element M^(a) in the formula (I) contains at least one ofSr and Ca. In the case where the element M^(a) contains at least one ofSr and Ca, the total molar ratio of Sr and Ca contained in the elementM^(a) is, for example, 85% by mol or more, and preferably 90% by mole

Preferably, from the viewpoint of stability of crystal structures, theelement M^(b) in the formula (I) contains at least Li. In the case wherethe element M^(b) contains Li, the molar ratio of Li contained in M^(b)is, for example, 80% by mol or more, and preferably 90% by mol.

The values of parameters v, w and x in the formula (I) are notspecifically limited so far as they each satisfy the above-mentionednumerical value range. From the viewpoint of the stability of crystalstructures, the parameter v is preferably in a range of 0.80 or more and1.05 or less, more preferably in a range of 0.90 or more and 1.03 orless. The parameter w is, from the viewpoint of stability of crystalstructures, preferably in a range of 0.80 or more and 1.05 or less, morepreferably in a range of 0.90 or more and 1.03 or less. The parameter xis an activation amount of at least one element M^(c) selected from thegroup consisting of Eu, Ce, Tb and Mn, and may be appropriately selectedso as to attain the desired characteristics. Preferably, the parameter xis a number satisfying 0.001<x≤0.020 and is even more preferably anumber satisfying 0.002≤x≤0.015.

First Film

The nitride fluorescent material has a fluoride-containing first film,and the first film contains the metal element M2. Preferably, the firstfilm has a first layer and a second layer from the side of thefluorescent material core, and the first layer contains a fluoridehaving a composition differing from that in the second layer. The firstlayer and the second layer of the first film may function as adouble-layer protective film to prevent the fluorescent material corefrom degrading in external environments, and the durability of a lightemitting device containing the nitride fluorescent material can bethereby improved.

In the first film of the nitride fluorescent material, the mechanism offorming the first layer and the second layer from the side of thefluorescent material core is not clear, but it is considered that, bybringing the calcined body into contact with a fluorine-containingsubstance followed by the first heat treatment in a temperature range of100° C. or higher and 500° C. or lower, the element M^(a) constitutingthe skeleton of the crystal structure of the fluorescent material coreformed of the calcined body may react with fluorine to form afluoride-containing second layer on the side of the surface of thefluorescent core. Further, it is presumed that, through the first heattreatment, and depending on the oxidation condition of the surface ofthe calcined body to be the fluorescent material core and on crystalstructure condition of the calcined body to be the fluorescent materialcore, for example, the presence of lattice defects as well as the amountthereof, fluorine in the fluorine-containing substance may react insidethe crystal structure of the calcined body to thereby form the firstlayer that contains the element M^(a), Al, fluorine and nitrogen.Preferably, the fluoride contained in the first layer has a compositionhaving a larger molar ratio of Al relative to the element M^(a) thanthat of the fluoride contained in the second layer.

It is presumed that, the fluoride contained in the first layer has acomposition having a larger molar ratio of Al relative to the elementM^(a) than that of the fluoride contained in the second layer, thus, thefluorescent material core and the first layer may strongly bond to eachother.

In the case where the second film is formed on the calcined body havingthe first film and then subjected to second heat treatment, thecomposition of the first layer and the second layer in the first filmmay change depending on the metal oxide contained in the second film. Onthe second layer arranged on the side of the second film in the firstfilm, oxygen in the metal oxide contained in the second film acts sothat the metal oxide contained in the second film reacts with thefluoride contained in the first film, and the second layer contains afluoride containing fluorine, the element M^(a) and Al, the metalelement M2, and oxygen in an amount larger than that in the first layer.It is presumed that, in the fluoride contained in the second layer, theelement M^(a) or Al constituting the fluoride may bond to the metaloxide contained in the second film via oxygen to form a stablestructure.

In the first film, the first layer on the side of the fluorescentmaterial core receives less action of oxygen in the metal oxidecontained in the second film than the second layer and thereforecontains a fluoride containing fluorine, Al, and the element M^(a), andoxygen in an amount smaller than that in the second layer. The firstlayer on the side of the fluorescent material core may contain anextremely minor amount of the metal element M2, as the case may be. Itis presumed that the first layer of the first film and the second layerof the first film may contain elements, specifically the elements of theamounts shown in Table 2 given in the section of Examples as describedhereinunder.

In the nitride fluorescent material, preferably, the fluorine content ismore than in a range of 0.5% by mass and 4.5% by mass or less relativeto 100% by mass of the nitride fluorescent material, more preferably ina range of 1.0% by mass or more and 4.0% by mass or less. When thefluorine content falls within the above-mentioned range relative to 100%by mass of the nitride fluorescent material, the fluoride-containingfirst film can function as a protective film for contributing towardpreventing the fluorescent material core from degrading in externalenvironments.

Second Film

The nitride fluorescent material further has, on the surface of thefluorescent material core, a second film containing a metal oxide thatcontains at least one metal element M2 selected from the groupconsisting of Si, Al, Ti, Zr, Sn and Zn. In the nitride fluorescentmaterial, preferably, the second film contains fluorine and the amountof fluorine contained in the first film is larger than the amount offluorine contained in the second film. The nitride fluorescent materialhas the fluoride-containing first film and the metal oxide-containingsecond film in that order from the side of the fluorescent materialcore. And further, the nitride fluorescent material preferably has thefirst layer of the first film, the second layer of the first film andthe second film in that order from the side of the fluorescent materialcore. The nitride fluorescent material has the first film and the secondfilm in that order from the side of the fluorescent material coretherein, in which, therefore, the elements constituting the crystalstructure of the fluorescent material core more hardly react with carbondioxide or moisture even in environments at relatively high temperatureand humidity. As a result, the light emitting device using the nitridefluorescent material of one embodiment of the present disclosure can bemore excellent in durability even in environments at relatively hightemperature and humidity. In addition, the nitride fluorescent materialhas the first layer of the first film, the second layer of the firstfilm and the second film in that order from the side of the fluorescentmaterial core, in which, therefore, the fluorescent material core isprotected by the three-layer structure, and the fluorescent materialcore is hardly influenced and degraded by external environments. And thedurability of the light emitting device using the nitride fluorescentmaterial can be thereby improved more.

Preferably, in the nitride fluorescent material, the oxide-equivalentcontent of the metal element M2 is 1% by mass or more and 20% by mass orless relative to 100% by mass of the nitride fluorescent material, morepreferably 2% by mass or more and 15% by mass or less even morepreferably 3% by mass or more and 12% by mass or less. When theoxide-equivalent content of the metal element M2 falls within theabove-mentioned range relative to 100% by mass of the nitridefluorescent material, the second film can contain a sufficient amount ofthe metal oxide to exhibit the function thereof for preventing carbondioxide and moisture existing in the external environments frompenetrating into the crystal structure of the fluorescent material core.

The metal element M2 contained in the metal oxide in the second film ofthe nitride fluorescent material is at least one element selected fromthe group consisting of Si, Al, Ti, Zr, Sn and Zn, more preferably atleast one element selected from the group consisting of Si, Al, Ti andZr, even more preferably at least one element selected from the groupconsisting of Si, Al and Ti, and especially preferably Si. When themetal element M2 contained in the metal oxide in the second film of thenitride fluorescent material is at least one element selected from thegroup consisting of Si, Al, Ti, Zr, Sn and Zn, the metaloxide-containing second film may be formed according to a sol-gelprocess using a metal alkoxide containing the metal element M2. Further,in the second heat treatment at higher than 250° C. and 500° C. orlower, dehydration may run on in the second film containing a hydroxylgroup (OH), and therefore a second film containing a large amount ofbonds (M2-O-M2) between the metal element M2 and oxygen can be formed.The metal oxide contained in the second film may react also with thefluoride containing fluorine, the element M^(a) and Al, which iscontained in the first film, in the second heat treatment, and thesecond film may contain fluorine and may further contain the elementM^(a) and Al.

Especially in the case where the metal element M2 contained in the metaloxide in the second film of the nitride fluorescent material is Si andwhere the second film of the nitride fluorescent material is formed, thesecond film containing silica (SiO₂) and partly containing a hydroxylgroup (OH) can be formed according to a sol-gel process using a silicaalkoxide. In the second film, dehydration runs on through the secondheat treatment, and the film contains a large amount of silicon-oxygenbonds (Si—O—Si), and further, the second film reacts with the fluoridecontained in the first film, and may therefore contain fluorine andfurther the element M^(a) and Al.

Preferably, the nitride fluorescent material absorbs light in awavelength range of 400 nm or more and 570 nm or less that is a range ofUV light to short wavelength side of visible light, and emitsfluorescence in a wavelength range where the light emission peakwavelength is 630 nm or more and 670 nm or less.

Regarding the light emission spectrum of the nitride fluorescentmaterial, the light emission peak wavelength thereof falls within arange of 630 nm or more and 670 nm or less, but is preferably within arange of 640 nm or more and 660 nm or less. The full width at halfmaximum of the light emission spectrum is, for example, 65 nm or lessand is preferably 60 nm or less. The lower limit of the full width athalf maximum is, for example, 45 nm or more.

In the nitride fluorescent material, the center of light emission is arare earth Eu, Ce or Tb, or a Group-7 Mn. However, The center of lightemission of the nitride fluorescent material is not limited to Eu, Ce,Tb or Mn, and for example, in the case where Eu is contained, a part ofEu may be substituted with any other rare earth metal or alkaline earthmetal for coactivation with Eu for use herein. For example, a divalentrare earth ion, Eu²⁺ can exist stably and emit light when a suitablebase body is selected.

The mean particle size of the nitride fluorescent material is, from theviewpoint of preventing light reflection and discoloration by the firstfilm and the second film and of attaining a desired light emissionintensity, for example, 4.0 μm or more, preferably 4.5 μm or more, andmore preferably 5.0 μm or more. The mean particle size of the nitridefluorescent material is, for example, 25.0 μm or less, and preferably22.0 μm or less. The mean particle size of the nitride fluorescentmaterial is the mean particle size of the nitride fluorescent materialhaving the first film and the second film.

When the mean particle size of the nitride fluorescent material is apredetermined level or more, the absorbance of excitation light by thenitride fluorescent material and the light emission intensity from thenitride fluorescent material may tend to be higher. In that manner, whenthe nitride fluorescent material excellent in light emissioncharacteristics is incorporated in the light emitting device to bementioned below, the light emission efficiency of the light emittingdevice can be high. In addition, when the mean particle size is apredetermined level or less, the workability in the production processfor the light emitting deice can be improved.

In this description, the mean particle size of the nitride fluorescentmaterial and the mean particle size of any others are the particle size(median size) measured using a volume-average particle size, laserdiffractometric particle size distribution measuring apparatus (productname: MASTER SIZER 2000, manufactured by Malvern Panalytical Ltd).

Light Emitting Device

Next, the light emitting device using the nitride fluorescent materialas a constituent element of the wavelength converting member therein isdescribed.

The light emitting device of one embodiment of the present disclosure isprovided with the nitride fluorescent material of the embodiment of thepresent disclosure and an excitation light source.

The excitation light source for use in the light emitting device ispreferably an excitation light source that emits light having awavelength in range of 400 nm or more and 570 nm or less. When theexcitation light source having the wavelength range is used, a lightemitting device having a high light emission intensity of thefluorescent material can be provided. The light emitting element to beused as the excitation light source for the light emitting devicepreferably has a main light emission peak wavelength in a range of 420nm or more and 500 nm or less, more preferably in a range of 420 nm ormore and 460 nm or less. When the light emitting element having the peaklight emission wavelength that falls within the above-mentioned range isused as the excitation light source, a light emitting device capable ofemitting a mixed light of light from the light emitting element andfluorescence from the fluorescent material can be constructed.

As the light emitting element, preferably, a semiconductor lightemitting element using a nitride semiconductor (In_(X)Al_(Y)Ga_(1-X-Y)N,0≤X, 0≤Y, X+Y≤1) is used. When the semiconductor light emitting elementis used as the excitation light source for the light emitting device, astable light emitting device having a high output linearity relative toinput with high efficiency and highly resistant to mechanical shock canbe obtained. The full width at half maximum of the light emissionspectrum of the light emitting element is, for example, preferably 30 nmor less.

The light emitting device includes at least the nitride fluorescentmaterial of the embodiment of the present disclosure. In the nitridefluorescent material, preferably, the fluorescent material core has acomposition shown by the formula (I), and is photo-excited with lighthaving in a wavelength range of 400 nm or more and 570 nm or less, andthe light emission peak wavelength thereof falls within a wavelengthrange of 630 nm or more and 670 nm or less. Preferably, the lightemitting device includes a first fluorescent material containing theabove-mentioned nitride fluorescent material, and a second fluorescentmaterial.

The first fluorescent material may be, for example, contained in thesealing member to cover the excitation light source to construct thelight emitting device. In the light emitting device where the excitationlight source is covered with the sealing member that contains the firstfluorescent material, a part of light emitted from the excitation lightsource is absorbed by the first fluorescent material and is emitted asred light. When the excitation light source capable of emitting light ina wavelength range of 400 nm or more and 570 nm or less is used, theemitted light can be more efficiently utilized.

The content of the first fluorescent material contained in the lightemitting device is not specifically limited, and may be appropriatelyselected in accordance with the color that are desired to be finallyobtained. For example, the content of the first fluorescent material maybe in a range of 1 part by mass or more and 200 parts by mass or lessrelative to 100 parts by mass of the resin to constitute the sealingmember, and is preferably in a range of 2 parts by mass or more and 180parts by mass or less.

The light emitting device may include a second fluorescent material thatdiffers from the first fluorescent material in point of the lightemission peak wavelength.

Examples of the second fluorescent material include fluorescentmaterials having a composition of any of the following formulae (IIa) to(IIi), and preferably, at least one fluorescent material having acomposition represented by any one selected from the group of theseformulae is contained. For example, from the viewpoint of attaining awide color reproduction range, more preferably, at least one fluorescentmaterial having a composition represented by the formula (IIc), (IIe) or(IIi) is contained. From the viewpoint of attaining a high colorrendering property, more preferably, at least one fluorescent materialhaving a composition represented by the formula (IIa), (IId), (IIf) or(IIg) is contained.

(Y,Gd,Tb,Lu)₃(Al,Ga)₅O₁₂:Ce  (IIa)

(Ba,Sr,Ca)₂SiO₄:Eu  (IIb)

Si_(6-p)Al_(p)O_(p)N_(8-p):Eu(0<p≤4.2)  (IIc)

(Ca,Sr)₈MgSi₄O₁₆(Cl,F,Br)₂:Eu  (IId)

(Ba,Sr,Ca)Ga₂S₄:Eu  (IIe)

(Ba,Sr,Ca)₂Si₅N₈:Eu  (IIf)

(Sr,Ca)AlSiN₃:Eu  (IIg)

K₂(Si,Ge,Ti)F₆:Mn  (IIh)

(Ba,Sr)MgAl₁₀O₁₇:Mn  (IIi)

Preferably, the mean particle size of the second fluorescent material isin a range of 2 μm or more and 35 μm or less, more preferably in a rangeof 5 μm or more and 30 μm or less. When the mean particle size is apredetermined level or more, the light emission intensity can beenlarged. When the mean particle size is a predetermined level or less,the workability in the production process for the light emitting devicecan be improved.

The content of the second fluorescent material may be, for example, in arange of 1 part by mass or more and 200 parts by mass or less relativeto 100 parts by mass of the resin constituting the sealing member, andis preferably in a range of 2 parts by mass or more and 180 parts bymass or less.

The content ratio of the first fluorescent material to the secondfluorescent material (first fluorescent material/second florescentmaterial) may be, for example, in a range of 0.01 or more and 5.00 orless as the ratio by mass, and is preferably in a range of 0.05 or moreand 3.00 or less.

The first fluorescent material and the second florescent material(hereinafter simply referred to as “fluorescent material”) mayconstitute the sealing member to cover the light emitting element alongwith a resin. The resin to constitute the sealing member includes asilicone resin, and an epoxy resin.

The total content of the fluorescent material in the sealing material toconstitute the sealing member may be, for example, in a range of 5 partsby mass or more and 300 parts by mass or less relative to 100 parts bymass of the resin, and is preferably in a range of 10 parts by mass ormore and 250 parts by mass or less, more preferably in a range of 15parts by mass or more and 230 parts by mass or less, and even morepreferably in a range of 15 parts by mass or more and 200 parts by massor less. When the content of the fluorescent material in the sealingmaterial falls within the above range, the wavelength of light emittedby the light emitting element can be efficiently converted by thefluorescent material.

The sealing member may further contain a filler, a light diffusingmaterial in addition to the resin and the fluorescent material. Forexample, when a light diffusing material is contained, thedirectionality from the light emitting element may be relaxed and theviewing angle may be thereby enlarged. Examples of the filler includesilica, titanium oxide, zinc oxide, zirconium oxide, alumina, etc. Inthe case where the sealing member contains a filler, the content thereofmay be appropriately selected depending on the intended object, etc. Thefiller content may be, for example, in a range of 1% by mass or more and20% by mass or less relative to the resin.

One example of the light emitting device of this embodiment is describedwith reference to the drawing. FIG. 1 is a schematic cross-sectionalview showing one example of the light emitting device of thisembodiment. The light emitting device is an example of a surface-mountlight emitting device.

A light emitting device 100 is provided with a package having a recesspart formed by lead electrodes 20 and 30 and a molded body 40, a lightemitting element 10, nd a sealing member 50 to cover the light emittingelement 10. The light emitting element 10 is arranged inside the recesspart of the package, and is electrically connected to the pair ofpositive and negative lead electrodes 20 and 30 provided on the moldedbody 40 via a conductive wire 60. The sealing member 50 is filled in therecess part to cover the light emitting element 10, and seals up therecess part of the package. The sealing member 50 contains, for example,a florescent material 70 for wave length conversion of light from thelight emitting element 10, and a resin. Further, the fluorescentmaterial 70 contains a first fluorescent material 71 and a secondfluorescent material 72. A part of the pair of positive and negativelead electrodes 20 and 30 is exposed out on the outside surface of thepackage. Via these lead electrodes 20 and 30, the light emitting device100 receives power from the outside to emit light.

The sealing member 50 contains the resin and the fluorescent material,and is formed to cover the light emitting element 10 arranged inside therecess part of the light emitting device 100.

EXAMPLES

Hereinunder the present invention is described more specifically withreference to examples, but the present invention is not limited to theseexamples.

Production of Calcined Body

A calcined body to be a fluorescent material core having a compositioncontaining Sr, Li, Eu, Al and N was produced. Specifically, forproducing a calcined body to be a fluorescent material core having acomposition represented by the formula (I) M^(a) _(v)M^(b) _(w)M^(c)_(x)Al_(3-y)Si_(y)N_(z), SrN_(u) (corresponding to u=2/3, mixture ofSr₂N and SrN), SrF₂, LiAlH₄, AlN, and EuF₃ were used as raw materials insuch a manner that M^(a) could be Sr, M^(b) could be Li, and M^(c) couldbe Eu. In this Example, the parameter y in the formula (I) is 0, and thefluorescent material core does not contain Si. The above-mentioned rawmaterials were weighed in a glove box in an inert gas atmosphere in sucha manner that a molar ratio of those as a charged amount ratio could beSr:Li:Eu:Al=0.9925:1.2000:0.0075:3.0000, and then mixed to give a rawmaterial mixture. Here, weight ratio of SrNu to SrF₂ was 94:6. Since Li(lithium) readily scatters during calcining, it was blended in an amountsomewhat larger than a theoretical value thereof. The raw materialmixture was filled in a crucible, and heat-treated therein in a nitrogengas atmosphere under a gas pressure of 0.92 MPa as a gauge pressure(1.02 MPa as an absolute pressure) and at a temperature of 1100° C. for3 hours to give a calcined body to be a fluorescent material core havinga composition represented by Sr_(0.9925)Li_(1.0000)Eu_(0.0075)Al₃N₄.Subsequently, the calcined body was dispersed and classified to give acalcined body 1.

Example 1 First Heat Treatment and Formation of First Film

The calcined body 1 was subjected to first heat treatment in anatmosphere containing a fluorine gas (F₂) and a nitrogen gas (N₂) andhaving a fluorine gas concentration of 20% by volume and a nitrogen gasconcentration of 80% by volume, at a temperature of 150° C. for atreatment time of 8 hours, and then in air, this was subjected to postheat treatment at a temperature of 300° C. for a treatment time of 10hours to give a fluorescent material precursor 1 formed of the calcinedbody 1 and a first film.

Formation of Second Film

100 g of the fluorescent material precursor 1 was put into a motherliquid prepared by mixing 180 ml of ethanol and 30 ml of aqueous ammoniacontaining 17.5% by mass of ammonia, and stirred, and the liquidtemperature was kept at 50° C. 85 g of tetraethoxysilane (Si(OC₂H₅)₄)was referred to as a liquid A, and a liquid prepared by mixing 23 ml ofaqueous ammonia having a concentration of 17.5% by mass and 23 ml ofpure water was referred to as a liquid B. While the mother liquid wasstirred, the liquid A and the liquid B were dropwise added to the motherliquid to prepare a mixed liquid, and after the dropwise addition of theliquid A and the liquid B, the mixed liquid was stirred for 30 minutes,then the stirring was stopped, a fluorescent material precursor composedof the calcined body 1, the first film and a second film was taken outof the mixed liquid, and dried at 105° C. for 3 hours to give afluorescent material precursor 2.

Second Heat Treatment

The fluorescent material precursor 2 was subjected to second heattreatment in air at a temperature of 300° C. for a treatment time of 10hours to give a nitride fluorescent material of Example 1. The nitridefluorescent material of Example 1 had the fluorescent material coreformed of the calcined body 1, the first film containing a fluoride, anda second film containing silica (SiO₂).

Example 2

The fluorescent material precursor 2 was subjected to second heattreatment in air at a temperature of 350° C. for a treatment time of 10hours to give a nitride fluorescent material of Example 2. The nitridefluorescent material of Example 2 had a fluorescent material core formedof the calcined body 1, a first film containing a fluoride, and a secondfilm containing silica (SiO₂).

Comparative Example 1

As it was, the calcined body 1 was a nitride fluorescent material ofComparative Example 1.

Comparative Example 2

The fluorescent material precursor 1 on which the second film was notformed was a nitride fluorescent material of Comparative Example 2.

Comparative Example 3

The fluorescent material precursor 2 not subjected to second heattreatment was a nitride fluorescent material of Comparative Example 3.

Comparative Example 4

The fluorescent material precursor 2 was subjected to heat treatment inair at a temperature of 250° C. for a treatment time of 10 hours to givea nitride fluorescent material of Comparative Example 4. The nitridefluorescent material of Comparative Example 4 had a fluorescent materialcore formed of the calcined body 1, and had, on the surface of thefluorescent material core, a film partly containing a fluoride, and asecond film containing silica (SiO₂). The nitride fluorescent materialof Comparative Example 4 had, on the surface of the fluorescent materialcore, a part in which the fluoride-containing film could not bedifferentiated from the second film.

Comparative Example 5

On the calcined body 1, a second film was formed in the same manner asin Example 1 without the first heat treatment and the first filmformation, thereby producing a nitride fluorescent material ofComparative Example 5.

Comparative Example 6

On the calcined body 1, a second film was formed in the same manner asin Example 1 without the first heat treatment and the first filmformation, and then this was subjected to second heat treatment in airat a temperature of 350° C. for a treatment time of 10 hours, therebyproducing a nitride fluorescent material of Comparative Example 6.

Evaluation of Nitride Fluorescent Material Light EmissionCharacteristics

The light emission characteristics of each nitride fluorescent materialwere measured. The light emission characteristics of the nitridefluorescent material were measured at an excitation light wavelength of450 nm using a spectral fluorophotometer (product name: QE-2000,manufactured by Otsuka Electronics Co., Ltd.). From the measured lightemission spectrum, the relative light emission intensity (%) of eachnitride fluorescent material was determined. The relative light emissionintensity (%) was calculated based on the relative light emissionintensity, 100% of the nitride fluorescent material of ComparativeExample 1. In addition, the chromaticity (x, y) of the fluorescentmaterial of each nitride fluorescent material was measured. The resultsare shown in Table 1. The light emission peak wavelength of the nitridefluorescent materials of Examples 1 and 2 and the nitride fluorescentmaterials of Comparative Examples 1 to 6 was all 650 nm to 660 nm. FIG.2 shows the light emission spectra of the nitride fluorescent materialsof Example 2, Comparative Example 1, Comparative Example 2 andComparative Example 4.

Composition Analysis 1

Each nitride fluorescent material was subjected to composition analysisaccording to ICP emission spectrometry using an inductively coupledplasma emission spectrometer (manufactured by Perkin Elmer Corporation).Regarding silicon (Si), the data obtained through composition analysiswere converted into oxide-equivalent data, and based on the nitridefluorescent material as 100% by mass, the content of silica (SiO₂) inthe second film was calculated. Regarding fluorine, when the contentthereof is less than 1.0% by mass, quantitative analysis was carried outusing an ion chromatograph (ICS-1500 manufactured by DIONEX Corporation)through ion chromatography, and when the content of fluorine is 1.0% bymass or more, quantitative analysis was carried out using a double beamspectrophotometer (U-2900 manufactured by HITACHI Limited) according toa UV-VIS method. Based on the nitride fluorescent material as 100% bymass, the fluorine content and the Si oxide-equivalent content weredetermined. Regarding the nitride fluorescent materials of ComparativeExamples 1 and 2, an Si-containing compound was not used as the rawmaterial for the calcined body and the second film was not formed, andtherefore the Si content was not determined. Regarding the nitridefluorescent materials of Comparative Examples 5 and 6, the first heattreatment was not carried out and the first film was not formed, andtherefore the fluorine content was not determined. The results are shownin Table 1.

Composition Analysis 2

The nitride fluorescent material of Example 2 was buried in an epoxyresin, the resin was cured, and then this was cut so that the crosssection of the nitride fluorescent material could be exposed out. Thesurface was polished with sandpaper, then finished with a cross sectionpolisher (CP), and the surface was photographed with a field emissionscanning electron microscope (FE-SEM, product name: JSM-7800F,manufactured by JEOL Ltd.). Thus photographed, the fluorescent materialcore, the first layer of the first film, the second layer of the firstfilm and the surface part of the second film of Example 2 were subjectedto composition analysis using an energy dispersive X-ray fluorescencespectrometer (EDX, product name: AZtec, manufactured by OxfordInstruments Corporation, acceleration voltage: 3 kV). The results areshown in Table 2. In Table 2, the composition of each of the first layerof the first film, the second layer of the first film and the secondfilm was expressed as a molar ratio on the basis of strontium (Sr) (1mol) contained in the fluorescent material core.

Mean Particle Size

Each nitride fluorescent material was analyzed using a laserdiffractometric particle size distribution measuring apparatus (productname: MASTER SIZER 2000, manufactured by Malvern Panalytical Ltd), andthe volume-average particle size (D50: median size) thus measured wasreferred to as the mean particle size.

SEM Image—Secondary Electron Image

Using a field emission scanning electron microscope (SEM, product name:SU3500, manufactured by Hitachi High-Technologies Corporation), SEMmicrographs of the nitride fluorescent materials of Comparative Examples1 to 3 and Example 1 were taken. FIG. 3 is a SEM micrograph of thenitride fluorescent material of Example 1; FIG. 5 is a SEM micrograph ofthe nitride fluorescent material of Comparative Example 1; FIG. 6 is aSEM micrograph of the nitride fluorescent material of ComparativeExample 2; and FIG. 7 is a SEM micrograph of the nitride fluorescentmaterial of Comparative Example 3.

SEM Image—Reflected Electron Image

The resultant nitride fluorescent material was buried in an epoxy resin,the resin was cured, and then this was cut so that the cross section ofthe nitride fluorescent material could be exposed out. The surface waspolished with sandpaper, and then finished with a cross section polisher(CP). Using a field emission scanning electron microscope (FE-SEM,product name: JSM-7800F, manufactured by JEOL Ltd.), SEM pictures of areflected electron image of a cross section of each nitride fluorescentmaterial of Example 2, Comparative Example 3 and Comparative Example 4were taken. FIG. 4 is a SEM micrograph of a reflected electron image ofa cross section of the nitride fluorescent material of Example 2; FIG. 8is a SEM micrograph of a reflected electron image of a cross section ofthe nitride fluorescent material of Comparative Example 3; and FIG. 9 isa SEM micrograph of a reflected electron image of a cross section of thenitride fluorescent material of Comparative Example 4.

Evaluation of Light Emitting Device Durability Evaluation

Using each nitride fluorescent material, a light emitting device wasconstructed. Using each nitride fluorescent material of Examples andComparative Examples as a first fluorescent material, and using a greenfluorescent material β-sialon as a second fluorescent material, anitride semiconductor light emitting element having a main wavelength of451 nm was sealed up with a sealing material dispersed in a siliconeresin, thereby constructing a surface-mount light emitting device havinga chromaticity (x, y)=(0.25, 0.22) or so. Each light emitting device wasstored in an environment tester at a temperature of 85° C. and arelative humidity of 85% for 300 hours and 500 hours to carry out adurability test. The x value on the chromaticity coordinate of the lightemitting device before the durability test was referred to as an initialvalue, and based on this, the absolute value of the difference betweenthe x values of the light emitting device after the durability test wasreferred to as Δx. The results are shown in Table 1. In the durabilitytest of the light emitting device using the nitride fluorescent materialof Comparative Examples 5 and 6, the chromaticity change after 300 hourswas already larger than in Comparative Example 1, or was on the samelevel as in Comparative Example 1, and therefore the durability test forthese nitride fluorescent materials were discontinued in 300 hours, andthe durability test after 500 hours was not carried out.

TABLE 1 Nitride Fluorescent Material Light Emission Light EmittingComposition Analysis Characteristics Device First Heat Second Heat MeanF SiO₂ Relative Light Durability Treatment Treatment Particle Size (% by(% by Chromaticity Emission 300 hrs 500 hrs Condition Condition D50 (μm)mass) mass) x y Intensity (%) Δx Δx Example 1 150° C., 300° C., 20.2 3.811.8 0.692 0.306 104 0.0004 0.0020 8 hrs 10 hrs Example 2 150° C., 350°C., 20.5 3.9 12.0 0.693 0.305 102 0.0002 0.0004 8 hrs 10 hrs Comparative— — 10.2 0.5 — 0.700 0.299 100 0.0245 0.0245 Example 1 Comparative 150°C., — 10.5 4.6 — 0.704 0.296 97 0.0015 0.0031 Example 2 8 hrsComparative 150° C., — 20.8 3.9 11.1 0.704 0.296 95 0.0177 0.0195Example 3 8 hrs Comparative 150° C., 250° C., 20.4 3.9 12.0 0.693 0.305104 0.0052 0.0107 Example 4 8 hrs 10 hrs Comparative — — 21.2 — 12.20.704 0.295 96 0.0279 — Example 5 Comparative — 350° C., 20.8 — 12.20.703 0.296 93 0.0218 — Example 6 10 hrs

TABLE 2 Composition (molar ratio) Sr Al N F O Si Example 2 FluorescentMaterial Core 1.0 2.9 3.2 − 0.2 − First First Layer 0.6 2.6 3.4 1.0 0.70.0 Film Second Layer 0.8 0.9 0.7 1.7 2.9 0.5 Second Film 0.1 0.2 0.10.5 4.1 1.5

As shown in Table 1, the light emitting devices using the nitridefluorescent material of Examples 1 and 2 have a smaller color change(Δx) after the durability test for 300 hours or 500 hours and wereexcellent in durability, as compared with the light emitting devicesusing the nitride fluorescent material of Comparative Examples 1 to 6.The nitride fluorescent materials of Examples 1 and 2 each have a firstfilm and a second film on the fluorescent material core, and it isconsidered that, owing to the double-layer protective film, thedurability could be improved.

As shown in Table 2, the nitride fluorescent material of Example 2 hadthree layers of the first layer of the first film, the second layer ofthe first film and the second film from the side of the fluorescentmaterial core. In the first film, the first layer on the fluorescentmaterial core side contained a fluoride having a composition containingSr, fluorine and Al, and further containing nitrogen, and furthercontaining oxygen in an amount smaller than that in the second layer.The fluoride contained in the first layer had a composition having alarger molar ratio of Al to Sr than that of the fluoride contained inthe second layer.

In the first film, the second layer arranged on the side of the secondfilm contained Sr in an amount larger than that in the first layer,fluorine in an amount larger than that in the first layer, and Al in anamount smaller than that in the first layer, and further containednitrogen in an amount smaller than that in the first layer and oxygen inan amount larger than that in the first layer. It is presumed that,since the amount of oxygen in the composition of the second layerarranged on the side of the second film is large, oxygen in silica(SiO₂) contained in a large amount in the second film may also act onthe second layer of the first film, and the second layer contains oxygenin an amount larger than that in the first layer, and Sr and Alconstituting the fluoride may bond to silica (SiO₂) in the second filmvia oxygen to form a stable structure, and therefore the second layer ofthe first film and the second film could firmly bond to each other.

As shown in Table 2, the second layer of the first film in the nitridefluorescent material of Example 2 further contained Si. The fluorinecontent in the first film was larger than the fluorine content in thesecond film, and the Si content in the first film was smaller than theSi content in the second film.

Also as shown in Table 2, the second film of the nitride fluorescentmaterial of Example 2 contained a large amount of silica (SiO₂) andfurther contained fluorine, Sr and Al, and contained nitrogen (N). It issuggested that, since the second film contained oxygen in an amount oftwo times or more of Si therein, the compound partly containing ahydroxyl group (OH) as formed according to a sol-gel process, wouldundergo dehydration, and therefore a film containing a large amount ofsilicon-oxygen bond (Si—O—Si) would be formed. It is presumed that thesilicon-oxygen bond (Si—O—Si) contained in the second film of thenitride fluorescent material of Example 2 could have reacted also withthe fluoride contained in the first film and could bond to Sr or Alconstituting the fluoride via oxygen, therefore forming a strong secondfilm. The second film contained fluorine along with silica (SiO₂), thefluorine content in the second film was smaller than the fluorinecontent in the first film, and the Si content in the second film waslarger than the Si content in the first film.

As shown in Table 1, the light emitting device using the nitridefluorescent material of Comparative Example 1 not having a first filmand a second film had a significant chromaticity change after the300-hours durability test, and it is known that the light emittingdevice was poor in durability.

In addition, in the light emitting device using the nitride fluorescentmaterial of Comparative Example 2 not having a second film, thefluorescent material core was protected by the first film, and thereforeeven after the 300-hours or 500-hours durability test, the chromaticitydid not change so much like in Comparative Example 1, but as comparedwith Examples 1 and 2, the chromaticity change after the durability testwas great, and it is known that the light emitting device was poor indurability.

As shown in Table 1, the light emitting device using the nitridefluorescent material of Comparative Example 3 had a larger chromaticitychange than the light emitting device using the nitride fluorescentmaterial of Comparative Example 2, and the durability thereof was low.The nitride fluorescent material of Comparative Example 3 had a firstfilm and a second film, but was not subjected to second heat treatmentafter the formation of the second film. Consequently, in the nitridefluorescent material of Comparative Example 3, the second filmcontaining silica (SiO₂) through hydrolysis and polycondensation oftetraethoxysilane partly contained a silicon compound having a residualhydroxyl group (OH) therein. Consequently, it is presumed thatdegradation of the nitride fluorescent material having low moistureresistance would be rather promoted and the fluorescent material corecould not be protected.

As shown in Table 1, the light emitting device using the nitridefluorescent material of Comparative Example 4 has a relatively smallchromaticity change as compared with the light emitting devices usingthe nitride fluorescent material of Comparative Examples 1 and 3, buthad a larger chromaticity change than the light emitting device usingthe nitride fluorescent material of Example 1 and the light emittingdevice using the nitride fluorescent material of Example 2, that is, thedurability thereof was not so much improved as that of the nitridefluorescent materials of Examples 1 and 2. The nitride fluorescentmaterial of Comparative Example 4 had a first film and a second film,and was subjected to the second heat treatment after the formation ofthe second film; however, it is presumed that, since the second heattreatment temperature was 250° C. and was low, a hydroxyl group (OH)would have remained in the second film and a second film containing alarge amount of silicon-oxygen bond (Si—O—Si) could not be formed, andthe protection of the fluorescent material core would be insufficient.

As shown in Table 1, the light emitting device using the nitridefluorescent material of Comparative Example 5 not having a first filmbut having a second film and not subjected to second heat treatment hada large chromaticity change than in Comparative Example 1 after the300-hours durability test, and the durability thereof was low. Thenitride fluorescent material of Comparative Example 5 did not have afirst film, and after formation of the second film thereon, this was notsubjected to second heat treatment, and accordingly, it is presumed thata part of the second film would have contained a hydroxyl group (OH) tothereby promote the degradation of the nitride fluorescent materialhaving low moisture resistance.

As shown in Table 1, the light emitting device using the nitridefluorescent material of Comparative Example 6 not having a first filmbut having a second film and was subjected to second heat treatment hasa chromaticity change on the same degree as that of Comparative Example1 after the 300-hours durability test, and the durability thereof waslow. The nitride fluorescent material of Comparative Example 6 did nothave a first film but was subjected to second heat treatment afterformation of the second film, however, the durability thereof could notbe so much improved like in Examples 1 and 2.

As shown in FIG. 2, the shape of the light emission spectra of thenitride fluorescent materials of Example 2, Comparative Example 1,Comparative Example 2 and Comparative Example 4 differs little, and itis presumed that the first heat treatment and the second heat treatmentwould not change the crystal structure of the fluorescent material core,that is, the crystal structure could be maintained as such after thetreatment.

The SEM micrograph of the nitride fluorescent material of Example 1shown in FIG. 3, and the SEM micrograph of the nitride fluorescentmaterial of Comparative Example 1 shown in FIG. 5 and the SEM micrographof the nitride fluorescent material of Comparative Example 2 shown inFIG. 6 differ in point of appearance. As shown in Table 1, the nitridefluorescent material of Example 1 has a larger mean particle size thanthat of the nitride fluorescent materials of Comparative Examples 1 and2, and therefore, it is presumed that the former particles would havesomewhat aggregated. From the appearance configuration shown in the SEMmicrograph of FIG. 3, it is presumed that the particles of the nitridefluorescent material of Example 1 would have somewhat aggregated, but asshown in Table 1, the light emitting device using the nitridefluorescent material of Example 1 was protected from chromaticity changeeven after the durability test.

As in the SEM micrograph of the cross section of the nitride fluorescentmaterial of Example 2 shown in FIG. 4, a nitride fluorescent material 1of Example 2 has a fluorescent material core 2, a first layer 3A of afirst film 3, a second layer 3B of the first film 3, and a second film4, are formed in that order from the side of the fluorescent materialcore. As shown in FIG. 4, when the first film is formed on the calcinedbody to be the fluorescent material core, then the second film is formedand thereafter subjected to the second heat treatment, it is presumedthat the second film containing a large amount of silicon-oxygen bond(Si—O—Si) could be formed, and further, oxygen contained in the secondfilm would act also on the first film, and it is presumed that the firstlayer and the second layer each having a different composition would beformed in the first film so that the fluorescent material core could beprotected by the three layers of the first layer, the second layer andthe second film.

The SEM micrograph of the nitride fluorescent material of ComparativeExample 3 shown in FIG. 7 differs in appearance from the SEM micrographsof the nitride fluorescent material of Comparative Example 1 shown inFIG. 5 and the nitride fluorescent material of Comparative Example 2shown in FIG. 6, and as shown in Table 1, the mean particle size of thenitride fluorescent material of Comparative Example 3 is larger than themean particle size of the nitride fluorescent materials of ComparativeExamples 1 and 2, and therefore it is presumed that the particles of thenitride fluorescent material of Comparative Example 3 would havesomewhat aggregated.

As in the SEM micrograph of the reflected electron image of the crosssection of the nitride fluorescent material of Comparative Example 3shown in FIG. 8, a nitride fluorescent material 5 of Comparative Example3 has a fluorescent material core 6 and has a fluoride-containing film 7and a second film 8 as formed partly on the surface of the fluorescentmaterial core 6. The nitride fluorescent material 5 of ComparativeExample 3 has a part in which the fluoride-containing film 7 could notbe clearly distinguished from the second film 8.

As in the SEM micrograph of the cross section of the nitride fluorescentmaterial of Comparative Example 4 shown in FIG. 9, a nitride fluorescentmaterial 11 of Comparative Example 4 has, partly on the surface of afluorescent material core 12, a fluoride-containing film 13 and a secondfilm 14 formed on the surface of the fluorescent material core 12 inthat order from the side of the fluorescent material core 12. Regardingthe nitride fluorescent material of Comparative Example 4, after thefluoride-containing film 13 was formed, the second film was formed andwas subjected to the second heat treatment, in which, however, thesecond heat treatment temperature was 250° C. and was low, and thereforeoxygen contained in the second film 14 could not sufficiently act on thefluoride-containing film 13, and consequently in the SEM micrograph ofthe cross section of the nitride fluorescent material, there is seen apart where the fluoride-containing film could not be clearlydistinguished from the silica-containing second film.

From the results shown in Table 1, the light emitting devices using thenitride fluorescent material of the present disclosure was moreprevented from chromaticity change after the durability test than inComparative Examples. According to the embodiment of the presentdisclosure, there can be provided a light emitting device excellent indurability.

The light emitting device using the nitride fluorescent material of theembodiment of the present disclosure is favorably used for light sourcesfor illumination, LED displays, backlight sources for liquid crystaldisplays, traffic lights, illumination switches, various sensors,various indicators, etc.

The invention claimed is:
 1. A nitride fluorescent material comprising,on the surface of a fluorescent material core having a compositioncomprising at least one element M^(a) selected from the group consistingof Sr, Ca, Ba and Mg, at least one element M^(b) selected from the groupconsisting of Li, Na and K, at least one element M^(c) selected from thegroup consisting of Eu, Ce, Tb and Mn, and Al, and optionally Si, and N,a first film comprising a fluoride, and a second film comprising a metaloxide that comprises at least one metal element M2 selected from thegroup consisting of Si, Al, Ti, Zr, Sn and Zn, wherein: the first filmcomprises at least one metal element M2 selected from the groupconsisting of Si, Al, Ti, Zr, Sn and Zn, and an amount of the metalelement M2 contained in the second film is larger than an amount of themetal element M2 contained in the first film.
 2. The nitride fluorescentmaterial according to claim 1, wherein the fluorescent material core hasa composition represented by the following formula (I):M^(a) _(v)M^(b) _(w)M^(c) _(x)Al_(3-y)Si_(y)N_(x)  (I) wherein M^(a)represents at least one element selected from the group consisting ofSr, Ca, Ba and M^(b) represents at least one element selected from thegroup consisting of Li, Na and K, M^(c) represents at least one elementselected from the group consisting of Eu, Ce, Tb and Mn, v, w, x, y andz each independently represent a number satisfying 0.80≤v≤1.05,0.80≤w≤1.05, 0.001<x≤0.1, 0≤y≤0.5, and 3.0≤z≤5.0.
 3. The nitridefluorescent material according to claim 1, wherein the metal element M2contained in the first film comprises Si, and the metal element M2contained in the second film comprises Si.
 4. The nitride fluorescentmaterial according to claim 1, wherein the second film comprisesfluorine, and an amount of fluorine contained in the first film islarger than an amount of fluorine contained in the second film.
 5. Thenitride fluorescent material according to claim 1, comprising the firstfilm and the second film in that order from the side of the fluorescentmaterial core.
 6. The nitride fluorescent material according to claim 1,wherein the first film comprises a first layer and a second layer inthat order from the side of the fluorescent material core, and the firstlayer and the second layer each comprise a fluoride having a differentcomposition.
 7. The nitride fluorescent material according to claim 6,wherein the first layer and the second layer each comprise a fluoridehaving a composition that comprises fluorine and Al, and the fluoridecontained in the first layer has a composition having a larger molarratio of Al relative to the element M^(a) than that in the compositionof the fluoride contained in the second layer.
 8. A light emittingdevice comprising the nitride fluorescent material of claim 1, and anexcitation light source.